Butterfly valves, in general, are well known and widely employed due to their simplicity of construction and relatively inexpensive cost, as compared to more complicated and detailed plug and ball valves. A typical butterfly valve generally comprises a disc mounted for rotation between the open position, in which the disc lies substantially parallel to the axis of the fluid flow channel through the valve, and the closed position, in which the disc lies perpendicularly to this axis. The disc is mounted for rotation on a valve stem or shaft, which is attached to the disc on one side. The disc cooperates with an annular flexible seat circumscribing the fluid flow channel for the purpose of effecting a resilient seal against the disc to shut off fluid flow through the channel. The annular flexible seat is conventionally held in position by being clamped in a recess formed between complementary surfaces of a portion of the valve body and a valve seat retainer.
The need for a certain degree of resilience and, thus, displaceability of the valve seat necessitates the use of an elastomeric material. If thermal or fluid pressure stresses cause distortions to the valve seat that could inhibit complete sealing, the elastomeric material is capable of distorting, so as to deform into the shape of the peripheral surface of the disc and establish a seal. However, materials of this type have a tendency to creep or migrate when subjected to high pressure, particularly when the pressure is applied to the seat on one side of the disc without a corresponding supporting pressure on the other side of the disc. Some conventional valve seats incorporate a reinforcing member in the seat in order to control this migration or creep without sacrificing the necessary resilience of the elastomeric material. The reinforcement is conventionally an annular ring or band of rigid material embedded within the seat as the seat is molded. An example of such a valve seat is disclosed in U.S. Pat. No. 3,940,108, to Edwards.
Other valve seats made of an elastomeric material have been provided that are configured to control migration or creep by configuring the seat to increase sealing pressure as a result of the pressure applied to the seat on one side. For example, U.S. Pat. No. 4,331,319 to Summers et al. discloses a valve seat having a U-shaped cross-section which provides surfaces to enhance the sealing effectiveness of the valve as a result of line pressure, regardless of the direction of application. However, the U-shaped valve seat requires a special groove formed into the valve body in order to compensate for the specific U-shaped configuration. This groove requires additional machining to the valve body and may further introduce additional stresses on the body while requiring a thicker overall body. In addition, although the U-shaped valve seat provides for some flexibility and resiliency in the radial direction, that flexibility may be limited, causing the valve seat to wear substantially during use as the seat rubs against the disc during opening and closing cycles.